1. Field of the Invention
The present invention relates to imaging systems. More specifically, the present invention relates to image trackers.
2. Description of the Related Art
Star sensors, wavefront sensors, target tracking and other image tracking devices often utilize an array of charge coupled devices (CCDs). These devices measure the location of an image spot on a picture element (pixel) of the CCD array with subpixel accuracy. Unfortunately, conventional devices are subject to numerous error sources. One such error source is known as a xe2x80x9csystematic errorxe2x80x9d or xe2x80x9cS-curvexe2x80x9d error. Systematic error appears as an aliasing effect when a centroid is computed from a waveform reconstructed from sampled pixelized input data.
One of the prior approaches to the systematic error is to add a systematic (e.g., spherical) aberration to the lens of the optical system. However, there are numerous shortcomings associated with this approach. First, the aberrations are generally wavelength dependent. Also, it tends to be an ad hoc approach adding considerable error to the image.
In xe2x80x9cElimination of Systematic Error in Subpixel Accuracy Centroid Estimationxe2x80x9d, published in the September 1991 issue of OPTICAL ENGINEERING, vol. 30, no. 8, pages 1320-1331, B. Alexander and K. Ng disclosed how systematic error occurs using a Fourier technique. Alexander and Ng determined that systematic centroiding error will occur if spatial frequencies are present above one cycle per pixel. Their approach to the elimination of those frequencies and the associated systematic error was to step down the aperture and increase the f-number of the associated optical system. However, one disadvantage of this approach is that if the aperture is down-sized, the sensitivity of the sensor will be degraded.
Hence, a need remains in the art for a system or technique for eliminating systematic error in the centroid determination of reconstructed waveforms from images generated by CCD image sensors.
The need in the art is addressed by the system and method of the present invention. The system and method of the invention substantially eliminates systematic error in a centroid determination of reconstructed waveforms from images generated by an image sensor. In accordance with the invention, a predetermined wavefront error is added to an input wavefront and the wavefront is detected. The predetermined wavefront error is effective to improve centroid determination.
In the illustrative embodiment, the input wavefront is passed through a random phase plate. The phase plate is an optical window in which the thickness in a z-axis varies randomly over an X/Y plane. The random phase plate acts as a low pass filter and the output of the phase plate is an aberrated wavefront. That is, the nonuniform thickness of the phase plate generates random spatial phase errors in the optical wavefront. The autocorrelation function of the phase plate is such that random phase errors in the optical wavefront will filter out spatial frequencies higher than one cycle per pixel. Hence, the systematic centroiding error is reduced.
In the illustrative embodiment, the aberrated wavefront is imaged onto a charge coupled device (CCD) detector by an optical arrangement. The optical arrangement may be implemented with either lenses or mirrors. The CCD is composed of discrete pixels which spatially sample the optical image and converts the photons in the blurred optical image into electrons. The electrons collected in each detector pixel are converted into a voltage by an analog signal processing circuit. An analog-to-digital (AID) converter converts the analog voltage to a digital signal. A digital circuit reformats the digital signal and provides an interface to a microprocessor. Software running on the microprocessor computes the position of the image centroid on the CCD using the digitized pixel data in a conventional manner.